Liquid crystal display devices using liquid crystal compositions are widely utilized for displays of watches, calculators, word processors and so on. The liquid crystal display devices utilize refractive index anisotropy, dielectric anisotropy and so on of liquid crystal compounds. As an operation mode in the liquid crystal display device, a mode mainly using at least one polarizing plate to display images is known, such as a phase change (PC), twisted nematic (TN), super twisted nematic (STN), bistable twisted nematic (BTN), electrically controlled birefringence (ECB), optically compensated bend (OCB), in-plane switching (IPS) or vertical alignment (VA) mode. Furthermore, a research has been recently conducted actively on a mode that exhibits electric birefringence by applying an electric field to an optically isotropic liquid crystal phase (Patent literature Nos. 1 to 14, Non-patent literature Nos. 1 to 3).
Furthermore, a proposal has been made for a wavelength variable filter, a wavefront control device, a liquid crystal lens, an aberration correction device, an aperture control device, an optical head device or the like utilizing electric birefringence in a blue phase as one of the optically isotropic liquid crystal phases (Patent literature Nos. 10 to 12).
The classification based on the driving mode of the device includes passive matrix (PM) and active matrix (AM) types. The passive matrix (PM) types are further classified into static type, multiplex type and so forth, and the AM types are further classified into thin film transistor (TFT) type, metal insulator metal (MIM) type and so forth.
The liquid crystal display devices include a liquid crystal composition having suitable physical properties. In order to improve characteristics of the liquid crystal display device, the liquid crystal composition preferably has suitable physical properties. General physical properties necessary for the liquid crystal compound as a component of the liquid crystal composition are as described below:
(1) being chemically stable and physically stable;
(2) having a high clearing point (clearing point: phase transition temperature between a liquid crystal phase and an isotropic phase);
(3) having a low minimum temperature of the liquid crystal phase (a nematic phase, a cholesteric phase, a smectic phase, an optically isotropic phase such a blue phase, or the like);
(4) having an excellent compatibility with other liquid crystal compounds;
(5) having a dielectric anisotropy of a suitable magnitude; and
(6) having a refractive index anisotropy of a suitable magnitude.
In particular, from a viewpoint of reducing the driving voltage, a liquid crystal compound having both a large dielectric anisotropy and a large refractive index anisotropy is preferred for the optically isotropic liquid crystal phase.
If a liquid crystal compound containing a liquid crystal compound being chemically and physically stable as described in property (1) is used in the liquid crystal display device, the voltage holding ratio can be increased.
With a liquid crystal composition containing a liquid crystal compound having a high clearing point or a low minimum temperature of the liquid crystal phase as described in properties (2) and (3), the temperature range of the nematic phase or the optically isotropic liquid crystal phase can be extended, and the liquid crystal composition can be used in the form of a display device in a wide temperature range. In order to develop characteristics that are difficult to be achieved by a single compound, the liquid crystal compound is generally used in the form of a liquid crystal composition prepared by mixing the compound with many other liquid crystal compounds. Accordingly, the liquid crystal compound to be used in the liquid crystal device preferably has the good compatibility with other liquid crystal compounds as described in property (4). In particular, a liquid crystal display device having a higher display performance in characteristics such as contrast, display capacity and response time has been recently required. Furthermore, a liquid crystal composition having a low driving voltage is required for a liquid crystal material to be used. Moreover, a liquid crystal compound having large dielectric anisotropy and refractive index anisotropy is preferably used in order to drive at a low voltage the optical device to be driven in the optically isotropic liquid crystal phase.